The present invention relates to a fixing device for an electrophotographic copier, comprising a heater plate having at least two layers and a surface over which a recording support carrying the toner image to be fixed is transported.
In electrophotographic copiers, a photoconductor layer is electrostatically charged and then exposed imagewise. The latent charge image which is thus produced on the photoconductor layer is developed into a visible image with the aid of a dry toner or a liquid toner. A liquid toner is substantially composed of a dispersing fluid comprising aliphatic hydrocarbons, wherein the toner is dispersed in the form of charged pigments. The developed, moist toner image is pressed against a recording support, for example, paper and is thereby transferred from the photoconductor layer to the recording support. The photoconductor layer is then cleaned for the next copying cycle. The moist toner image on the support, which is not yet fast to wiping, is fixed by the application of heat. A reliable technique which is used in many copiers consists in guiding the moist recording support over a heated plate, the support contacting the plate with its reverse side while being transported over the plate.
Such heater plates, which have a planar or curved shape, comprise a metal body with electrical heating elements embedded therein. In a known embodiment, the heater plate is constructed of cast aluminum and has the dimensions 2.5.times.85.times.290 mm, with two embedded heating rods each having 400 Watts heating power. The temperature is adjusted to about 200.degree. C. by means of a thermo-feeler, in a two-position control system, i.e., by switching on and off.
When conventional electrophotographic copiers are switched on after a prolonged downtime, the photoconductor is usually first cleaned from any dried-on toner residue by rinsing with toner liquid during a warming-up period of about 25 seconds. In this warming-up period, heating up of the heater plate to its working temperature commences. At the end of the warming-up period of 25 seconds, however, a heater plate of the above-described kind has a temperature of only about 65.degree. C., at an ambient temperature of about 22.degree. C. The working temperature of about 200.degree. C. is reached only after about 80 seconds. As a result, the first copies which are produced immediately after the warming-up period are often not fast to wiping after fixing. Particularly, in copiers operating at higher copying speeds of about 30 copies per minute and more, which have a collator for the copies connected to their output side, blurred copies are frequently found.
After a short downtime of the apparatus of up to some minutes, the heater plate which cools down slowly due to its high heat capacity, is still warm so that the warming-up period can be reduced to a few seconds and the waiting time for each copy can be kept short.
If the apparatus is additionally heated during the warming-up period, at a power which is far higher than the operating power, the normal admissible load limits of the electrical installations are often exceeded and there is consequently a risk of the apparatus breaking down.